When a well borehole is drilled to gain access to a prospective production zone, the original natural seal in the form of impermeable rock, termed cap rock, is disturbed. In abandoning the well, the seal must be reestablished to prevent the vertical migration of fluids through the well from the production zone. It is desirable that any borehole seal have the same sealing characteristics as the original seal.
During construction of a well, the drilled borehole is usually cased with steel. Often a cement sheath is placed about the casing to form a seal between the casing and the wall of the borehole. The conventional abandonment technology assumes permanent integrity of the casing to maintain the seal. Flow of fluid within the casing is controlled by setting a bridge plug in combination with cement plugs. Attempts to control flows outside the casing usually entail perforating the casing and injecting a cement slurry into the annulus. However, there are certain drawbacks associated with this procedure. For example, the force of the perforating charge and the pressure of the cement injection behind the casing can cause fracturing in the surrounding formation which may provide a path for fluid leaks about the plug. Further, because of the contamination of the borehole wall by the cement sheath and other substances such as oil, leakage can occur at the cement/formation and cement/casing interfaces. In addition, such a plug is not permanent since its integrity is reliant on the life of the steel casing, corrosion and disintegration of which will in itself create a conduit for future flow. It is desirable that a permanent plug be available for one-time abandonment.